It is well known that spectral sensitization or chemical sensitization is achieved by adding a pyrylium-based compound to a light-sensitive layer of an electrophotographic light-sensitive material.
Japanese Patent Publication No. 28499/65 discloses that a pyrylium-based compound and a thiapyrylium-based compound are added in combination with an organic compound such as a light-sensitive or light-insensitive resin, anthracene, anthraquinone and polyvinyl carbazole, or with an inorganic substance such as selenium to increase the sensitivity of the light-sensitive layer and to extend the spectral sensitivity and spectral range.
When a pyrylium-based compound is added to an electrophotographic photoreceptor, the light sensitivity of the photoreceptor is increased and the light-sensitive wavelength region is extended to longer wavelengths. It is believed that the above phenomenon occurs because a pyrylium compound absorbs light and generates an electric charge, specifically, the pyrylium compound absorbs light, becomes excited, and in this light excited condition, an electron migrates from a photoconductive substance, leading to generation of an electric charge. Extension of the wavelength region to longer wavelengths correlates to the absorption spectrum of a pyrylium-based compound.
In recent years, an electrophotographic photoreceptor of the laminate type wherein the function is separated into an electric charge-generating layer and an electric charge-transporting layer has been investigated. A layer in which an electric charge-generating organic pigment is dispersed is mainly used as the electric charge-generating layer. Various pigments, such as those having sensitivity to wavelengths ranging from visible light to the infrared ray region, or those having sensitivity only to visible light, or those having sensitivity only to infrared rays, have been proposed as electric charge-generating organic pigments. By properly selecting the electric charge-generating organic pigment, it is now possible to impart sensitivity in the desired spectral sensitive region.
The above laminate type electrophotographic photoreceptor using a pyrylium-based compound has been described in Japanese Patent Application (OPI) No. 88226/76. The term "OPI" as used herein means an "unexamined published patent application". An electrophotographic photoreceptor is disclosed in which an eutectic complex is formed by adding a pyrylium-based compound to a polymer having an alkylidenediarylene group as the repeating unit, e.g., a polycarbonate resin is used in an electric charge-generating layer.
The electrophotographic photoreceptor disclosed in this patent shows a spectral sensitive spectrum different from the spectral sensitive spectrum of the original pyrylium-based compound.
However, if the aforementioned pyrylium-based compound is added to the electric charge-generating layer for the purpose of increasing the light sensitivity of the electric charge-generating organic pigment, the resulting electrophotographic photoreceptor has a greatly increased rate of dark-damping. Thus, the electrification properties of the photoreceptor are reduced, and the photoreceptor is not suitable for practical use.